The integrity of a computer network""s physical layer is critical to its proper operation. While much attention is directed to the operation of the computer-nodes and the network communications devices, e.g., hubs and switches, the relevance assumes proper behavior at the physical layer. For example, in ubiquitous 10Base2, 5, T, and 100BaseT networks, a number of physical layer problems can lead to improper operation. Damage to the cabling can create impedance discontinuities in the conductors, which cause signal reflections. This can impair the proper signal transmission and decoding by the nodes or network communication devices. Further, length of the links can undermine the shared usage of the transmission media. Some protocol specifications, such as the inter-frame gap, are defined based upon the time needed for a given communication event to propagate throughout the entire communication network.
Time domain reflectometry (TDR) techniques have been used to analyze and validate computer networks at the physical layer. The basic process involves generating a predetermined, TDR, signal, such as an impulse or step function, on the conductors of the computer network. At the point of injection, a signal analysis device, such as a digital sampling oscilloscope, is used to monitor the computer network conductors for reflections induced by the TDR signal. These reflections are induced by impedance discontinuities along the computer network transmission media. The size of the reflected signals are indicative of the size of the impedance discontinuity, and the delay between the generation of the TDR signal and the detection of the reflection is indicative of the distance to the discontinuity based upon the round-trip signal travel time.
Generally, TDR is performed on non-operating networks. For example, when network cabling is newly installed, special terminators are placed at the end of links and the TDR device is attached to the link to inject the TDR signal. This allows the network to be verified at the physical layer, prior to connection of the computer network devices, such as the nodes, and the network communication devices, such as the hubs and/or switches.
Various techniques have been proposed for performing TDR analysis on operating networks. These existing techniques are typically targeted at 10BASE 2,5,T systems. In one proposed technique, the TDR signal is injected onto the link, to which operating computer nodes are attached. Simultaneously, the TDR system listens to determine whether the TDR signal has collided with inter-network device communication event. If such a xe2x80x9ccollisionxe2x80x9d has occurred, the TDR device forces a collision condition by driving the voltages on the network in accordance with the network""s protocol for declaring a collision. The collision condition is initiated to ensure that the network devices do not attempt to decode the corrupted transmission. Another proposed solution relies on the use of packet-like transmissions. Specifically, the TDR system generates a transmission that would appear to be a valid packet or frame transmission for the network. After the generation of the preamble, and possibly header information, rather than a payload, a TDR signal is transmitted. As a result, this system avoids a collision with other network devices by first asserting control of the communications link.
A problem with these existing techniques for performing TDR analysis is that they rely on the creation or existence of a silent period on the computer network link. For example, one of the previously described approaches injects the TDR signal and assumes that no communications are taking place on the network link. If communications were in fact taking place, a collision condition is forced. The other approach relies on first capturing the network link with a packet-like transmission and then terminating transmissions for the generation of the TDR signal. These approaches are generally acceptable for 10BASE(x) type network systems, where periods exist when there are no transmissions on the network. This assumption, however, can not be made for 100BASET networks. Even when no data is being exchanged between network devices, an idle, or recurrent, signal is transmitted.
In general, according to one aspect, the invention features a method for performing time domain reflectometry contemporaneously with recurrent transmissions, such as idle transmissions, on a computer network, such as a 100 BASET protocol network.
This method comprises detecting the recurrent transmissions and then storing representations of those transmissions. A probe signal, such as a TDR signal, is then generated onto the computer network during the recurrent transmissions. A response of the network to the probe signal combined with the recurrent transmissions is then detected. The signal transmission characteristics of the network are then analyzable based on the response to the probe signal using the stored representations of the recurrent transmissions.
In specific embodiments, the method also comprises detecting a response of the computer network to a probe signal while the computer network is inactive. This is used for providing a base line for future analysis. For example, the response of a network communications device, such as a hub or switch can be assessed and analyzed to facilitate analysis of a node-side of the link. In other aspects of the embodiments, the recurrent transmissions, which are stored, are idle signals from the network. The step of analyzing the signal transmission characteristics of the networks comprises removing the contribution of the idle signal to the detected combined response. This can be done using deconvolution.